TurboTax seems to have gone back and forth over the years on its treatment of a blank box 2a on a code-T Form 1099-R. My opinion is that TurboTax should not make an assumptions about how much is taxable and should treat the lack of an explicit entry of the taxable amount as an error. It's almost never the case that the distribution is fully taxable. It's quite common for the distribution to be nontaxable, but that's not guaranteed without gathering additional information, so TurboTax leaves it up to the recipient to determine the taxable amount.
